1. Field of the Invention
This invention relates to new and useful improvements in raise bore drill bits and more particularly to raise bore bits having a removable drive stem.
2. Description of the Prior Art
The use of raise bore drill bits for drilling vertical or angled shafts in mining is well-known in the art. Raise drilling comprises the drilling of a relatively small diameter pilot hole followed by enlargement of the hole to a predetermined raise diameter by rotating a raising a large diameter drill bit along the pilot hole to form a raise bore between a lower level and an upper level. The drill bit is rotated from above by a drill stem and guided by the pilot hole. During the drilling operation the cuttings produced fall through the drill to the lower level.
In raise bore drilling, raise bores are usually 48 inches in diameter or more and they range to sizes as large as 10 or 15 feet in diameter. The pilot hole will typically have a diameter of 9 to 15 inches. A raise drill bit is quite massive in size and may weigh 5,000 pounds or more. The raise drilling operation is commenced by rotation of the drill string by a drilling rig causing the raise drill bit to rotate at approximately 10 - 40 R.P.M. while the rig exerts an upward pull on the bit ranging from about 150,000 - 950,000 pounds. As the drilling progresses, sections of the drill stem which are usually cylindrical or tubular members approximately 5 feet in length are progressively uncoupled at the upper level and the raise is completed by rotating and lifting the drill stem and the raise drill bit.
It will be appreciated that such raise drill bits, drill stems and the drive connections to the drill bits are subjected to severe wear and stresses which ultimately result in failure, usually in fatigue. Certain of the prior art is concerned with the replacement of components, such as cutters and the like. Another problem in raise bore drilling involves the changing of the drill stem in the field after it has been broken or bent by dropping or cross-threaded connection or by fatigue failure. Also, limited space conditions in underground mines sometimes require a raise boring bit body to be at least partially disassembled so that it can be transported in small drifts, small cages, or through other small openings.